US2185894A - Method of producing malleable iron - Google Patents

Description

Patented Jan. 2, 1940 METHOD OF PRODUCING MALLEABLE IRON Axel Gustaf Emanuel Hultgren, Djursholm,

Sweden No Drawing. Application January 22, 1938, Se-

rial No. 186,463. In Sweden January 25, 1937 3 Claims.

According to the so-called American process malleable iron is produced from white cast iron of suitable composition in the following manner. The castings are first annealed at a temperature formed in the cementite, but in the martensite that constitutes a great part of the matrix after quenching, the remainder of the matrix being austenite. On heating-tempering-rnartensite above AC1, usually between 800 and 900 C. until it is usually decomposed into an aggregate of fer- 5 all free cementiteFesC-is consumed, i. e. unrite and cementite. In this case, i. e. on tempertil the original structure consisting of austenite ing hardened cast iron, of the composition comand cementite has been transformed into austenmonlyused for malleable castings, numerous subite and graphite, and are then allowed to cool microscopic graphite particles are also formed slowly, the temperature being usually kept mm which on continued heating to higher tempera- 10 stant, somewhat below An, for an extended tures grow to particles visible in the microscope. period. Owing to the slow cooling, the amount These particles act as nuclei for the precipitation of carbon that corresponds with the decreasing of graphite during the malleableizing anneal. solubility is precipitated as graphite, not as ce- Owing to the numerous graphite particles formed mentite. While the temperature is held below the malleableizing anneal, under favourable con- 16 An, the pearlite formed from austenite at Ari is ditions can be shortened to such an extent that decomposed into ferrite and graphite. Thus, the the whole process, including quenching, takes less end product, malleable iron, consists of ferrite than 12 hours. On quenching followed by rapid with particles of graphite distributed in it. For heating the number of graphite particles formed 20 the whole process several days are required. during malleableizing-as counted in unit area 20 Several years ago, two Japanese investigators, was multiplied more than 100 times as compared D. Saito and H. Sawamura, discovered the fact with the number formed on ordinary malleableizthat, on first quenching the-castings in water. ing treatment, If the heating after quenching from a high temperature, about 850-950 C., and was retarded the number was doubled again.

then tslilibjecting them to the malleableizing an- This is seen from the following figures: 25 neal, e number of graphite particles formed are o reatly increased and, in consequence the annealor} q 8 wlthout PIE-quenching treatment can be considerably shortened, say. pal 1c es P On quenching from 975 followed by rapid heatto less than 48 hours. This 18 described in in and malleableizing at C about 14 000 I Memoirs of the College of Engineering, Kyoto paiticlespm sq mm 30 Impeml 1927 On quenching from 975 (2., rapid heating to 600 under the title. A New Phenomenon Concerning C h o 1 olding at 600 C. for /2 hour and mallethe uraphitization of White Cast Iron and Its ableizm at C about 20 000 articles pro Application to the Manufacture of Black. Heart sq p Malleable Castings." The authors explain the o 35 phenomenon by assuming that the stresses proon quenching from 8 slow heatmg to 600 C., holding at 600 C. for hour and malleduced on the transformation of austenite into am izin t C about 30 000 articles m martensite cause the cementite particles to be e g a p p broken up into small fragments, in the free sur- 40 faces of which graphite nuclei are readily formed These results were obtained on 6 mm. rd. rods 40 during the subsequent annealing treatment. of white cast iron having the following comp Their hypothesis appears to be supported by the tion: 3.0% C, fact that the result described is obtained after 0.05% S. The rods were que n Oil and very rapidcooling only. The practical developthen'became martensiticment of this process, anticipated from the re- If this proc is pplied to castin f whi is duced cost of 'manufacture, has not, however, cast iron of ordinary analysis similar to the above been realized. and having the dimensions usually required for The present invention developed out of an inmalleable castin it fails for the following renvestigation carried out by the inventor with 9. sons: eith r q hing is doneinoil, 111, whi

view to ascertain the conditions required for case the matrix becomes sorbitic and the graphite 5 shortening the malleableizing anneal by a previparticles formed 11 nn lin are not increased ous quenching treatment. It was found that the in numb r (2) quenching is (10118 in in hypothesis put forth by Saito and Sawamura for which case quenching cracks cannot be avoided. explaining the formation of numerous graphite Thus, in order that the discovery ma er finuclei is not correct. ese nuclei are not ing the imiwrtanceo an. nslte asatrix for enhanced graphite nucleation maybe put to practical application the following conditions must be fulfilled.

(1) The cast iron should have such a composition that castings of the dimensions and'shape required can be made largely martensitic by quenching without the formation of quenching cracks. By the use of suitable alloy additions, such as Mn, Ni, Cr, Mo etcetera, the hardening depth of steel and cast iron can be raised to such an extent that even heavy objects may be through-hardened on quenching in oil or even on cooling in air, thus preventing the formation of quenching cracks.

(2)- The composition should be balanced in regard to tendency for graphitization, i, e. the castings, of required size and under existing casting conditions, should solidify white, with prac tically no graphite formation but graphite nuclei should readily form on tempering of the martensite and their growth on annealing should not be retarded or suppressed. C, Si, Ni and Cu favour graphitization, Mn, Cr and Mo retard it.

A common composition for making malleable iron is mentioned above. This is balanced in the sense of condition (2), i. e. the C and Si contents will, in the presence of the low Mn content, produce the desired graphitization tendency. The changes required in order to fulfil condition (1) canbe brought about in different ways, for instance:

After the various ways of' carrying out the invention have thus been defined in principle, the

actual amounts of the respective alloy contents to be applied in each particular case are easily found by experiment, at the same time taking into consideration the cost and'physical and chemical properties of the malleable iron produced.

The process for producing malleable iron for which patent is herewith applied consists in taking castings of a cast iron containing certain alloy components, on one hand at least one of such components as favour the graphitization, for instance, Si,-Ni, Cu, on the other at least one of such alloy components as increase the depth of hardening on quenching or cooling, for instance, Ni, Cr, Mn, in such concentrations that with existing dimensions and casting conditions, the castings solidify white and, further, can be hardened to suflicient depth to a largely martensiticaustenitic state by cooling from a temperature exceeding A1 in oil or equivalent or less eflicacious cooling medium, and subjecting said castings first to cooling in a cooling medium at most as efficacious as oil from a temperature exceeding A1 so as to transform the matrix into a largely martensitic-austenitic state, and then to a malleableization or cooled at a faster rate than ordinarily during the later stage of the malleableizing anneal. In making pearlitic malleable iron alloy cast iron is often employed. In that case, the object of the alloying additions is different from that of the present inventionand the heat-treatment succeeds malleableization instead of preceding it as described above. On the treatment may also be applied to castings that have been manufactured or treated according to the present invention, with the object of obtaining greater strength. The heat-treatment may consist in fairly rapid cooling from a temperature above A1 to give a pearlitic or sorbitic matrix or in quenching in oil or the like from a temperature above A1 to give martensitic structure followed by tempering to a temperature below A1. When abrasive resistance combined with the lubricating properties of graphite are called for tempering may be omitted or carried out at a low temperature.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. Method of producing malleable iron from cast iron, comprising making castings of white cast iron, containing a component increasing the depth of hardening taken from a group consisting of nickel, chromium and manganese, said component being in such concentration that the other hand a subsequent or concluding heatcastings will be hardened to suflicient depth to a largely martensitio-austenitic state by cooling from a temperature exceeding A1 in a cooling medium not more efficacious than oil, and subjecting the castings to such a cooling and then to a malleableizing anneal.

2. In a method claimed in claim 1, heating the heat-treated object, prior to the malleableizing anneal, up to about 600 C. at a rate not exceeding 20 per minute.

3. In a method claimed in claim 1, subjecting the heat-treated object to heating prior -to the malleableizing anneal, the object being kept within a range of temperature of between 550 and 700 for more than 10 minutes.

AXEL GUSTAF HULTGREN.